Efficiency of a photovoltaic system (power output from the photovoltaic system under the same irradiance) significantly changes depending on a use condition thereof such as a voltage used when the power is output from the photovoltaic system (FIG. 9).
Therefore, in order to fully use energy from the photovoltaic system, it is necessary to maintain a condition in which the power obtained from the photovoltaic system is maximized. For this purpose, a device for detecting and tracking a maximum power point of the photovoltaic system is necessary. However, this device is typically complicated and expensive, so that a total cost of the power source apparatus increases unfortunately. In addition, a method of detecting and tracking the maximum power point of the prior art such as a so-called “hill-climbing method” causes a power loss.
If the employed photovoltaic system is restricted (if characteristics of the employed photovoltaic system are known in advance), a voltage range for obtaining the maximum efficiency is also restricted. Therefore, it is possible to obtain the highest efficiency operation at a constant voltage without employing the hill climbing method or the like. Even in this case, an additional power circuit is necessary to adjust and convert the power, and this causes an increased coast and decreased efficiency. In this regard, if the power adjustment and conversion can be implemented without employing the power circuit, it is possible to remarkably reduce cost and improve efficiency.